The fabric used for bags, shoes, conveyors, etc., is manufactured by a weaving machine and has a periodic pattern such as lattice--or interdigital-stripes. Here it is to be noted that various defects unavoidably occur to a certain extent in these fabrics depending upon the homogeneity of those materials, working conditions, soiled spots in the manufacturing process.
On the other hand, other products having a periodic pattern are, for example, a shadow mask used in color CRTs and a liquid crystal panel used for liquid crystal display. The shadow mask is used as a color separation mechanism for the color CRT and comprised of an alloy plate with a periodic array of many fine openings for a plurality of color components provided to allow passage of electron beams of respective color components. The liquid crystal panel is so constructed as to have a matrix array of many pixels. These products, such as a shadow mask and liquid crystal panel, sometimes suffer various defects in the manufacturing process.
The checking of defects in the products with a periodic pattern has been made by human eye.
In recent years, a defect checking apparatus has been developed which automatically checks defects by taking an image from an object with a periodic pattern by means of a camera using a solid-state image pickup device such as a OCD image sensor and performing processing on an image signal obtained.
Taking the fabric as an example of an object, checking of a defect of a relatively large contrast, such as hole openings, burning marks, can be effected relatively easily on the conventional defect checking apparatus. However, it has been difficult to check microdefects specific to the fabric. That is, if the pixel pitch of the image pickup device is made finer to accurately check such a microdefect, then a moire pattern emerges as the size of the pixel gets nearer to a lattice of a periodic pattern of the object. If such a moire pattern appears, it becomes difficult to check the object for defects and this provides a bar to the detection of them.
Even on a TV camera such as a business TV camera for broadcasting in particular, a high quality video camera for a general consumer and an HDTV (high definition television) camera, if an image is taken from an object including a striped fabric having a fine periodic pattern, a moire interference emerges.
In order to avoid such a moire pattern, a conventional method is known by which the high component of a spatial frequency involved is cut off by inserting an optical lowpass filter in an incident light path of the image pickup device. However, this method involves a lowering in image resolution. That is, with the optical lowpass filter inserted, very fine information of the high component of the spatial frequency is sacrificed so that such a camera cannot be applied to a defect checking apparatus of high precision adapted to check such a microdefects as set out above. For the TV camera, the lowering of the resolution would lead to the degeneration of an image quality.
It is accordingly an object of the present invention to provide an image pickup apparatus applicable to a defect checking apparatus and TV camera capable of effectively eliminating moire interference without sacrificing an image resolution.